Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 2,235	$ 2,497
Current portion of deferred rent and lease incentive	0	117
Collaboration and licensing	1,050	1,222
Accrued patent fees	487	736
Accrued external research and development costs	4,379	5,344
Accrued professional and consulting services	446	994
Accrued grant refund	0	175
Accrued interest	82	106
Issuance costs, December financing	4,381	0
Other	432	509
Accrued expenses	$ 13,492	$ 11,700